In order to increase the power reserve, by increasing the number of turns of a mainspring, one solution consists in decreasing the diameter of the barrel arbour and of the associated core, so as to increase the space available for the spring inside the drum.
The ratio of the core radius to the spring thickness is usually comprised between 10 and 20 and the invention proposes to reduce this ratio to below 10, and preferably to within a range of between 5 and 10.
The sizing must not be too small; there is a risk of breakage if the core diameter is too small.
In the conventional barrel architecture, a ratchet is axially mounted on a barrel arbour or on a core, via a square, with the ratchet usually being secured by an axial screw. The dimension of this screw and that of the square thus define the minimum diameter of a pivot shoulder. A step portion joined to this pivot shoulder limits the endshake of the arbour or of the core relative to a bottom plate or to a bridge carrying a jewel or similar element.
An even larger diameter than that of the step portion is required for a shoulder for guiding the pivoting of the drum on the arbour or on the core, combined with a step portion limiting the endshake of the drum. The dimension chain required to observe minimal sections of material results in substantial dimensions, which are difficult to reduce. In particular, it is not sufficient merely to reduce all of the dimensions, since the cross-sections of material are then insufficient to ensure fatigue resistance.
The precondition for minimising the diameter of the spring-holder core and the diameter of the barrel arbour is limiting any shake as far as possible, on the one hand between the drum and the core, and on the other hand the shake of the core (or the arbour) relative to the bridges. Shake must naturally be adjusted to allow for any expansion or contraction linked to variations in temperature within the range of use of the watch. The barrel architecture must thus allow for compatibility between shakes, with the lowest possible values, while preventing or at least limiting unwanted and energy consuming friction.
The design of a barrel is connected to generally large scale production, and manufacturing costs must be kept at the lowest possible level, with machining operations simplified as much as possible. Naturally, after-sales requirements mean that the barrel must be easy to dismantle, particularly in order to change the spring.
FR Patent No 306103 in the name of Etablissements Parrenin discloses a hollow core, whose length is equal to the thickness of the barrel, in which a longer, removable arbour pivots in a plate and a bridge. The core and the arbour have complementary pivotal drive means, via a square, yet relative axial motion is allowed between the core and arbour, one of the ends of the arbour including a stepped square driving a ratchet. The ratchet is held outside the barrel, beyond the bridge. A drum pivots on a stepped shoulder of the core, and a cover fixed to the drum pivots on another stepped shoulder of the core. The drum shake relative to the core is limited by two stepped portions of these two stepped shoulders in opposite directions. The core shake relative to the arbour is limited, on the ratchet side, by a stepped portion of the arbour cooperating with an end face of the core, while at the other end the other end face of the core bears on the plate. Limiting the shake of the arbour relative to the plate and to the bridge is achieved in one direction, either indirectly, by the core cooperating with the stepped portion of the arbour, or by the ratchet bearing on the bridge, while in the other direction it is necessary to add, beyond the end of the arbour provided with the ratchet drive square, a removable stop member, or an extra bridge, which has the drawback of increasing the necessary thickness inside the movement.
CH Patent No 324249 in the name of COMPAGNIE DES MONTRES LONGINES, FRANCILLON S.A. discloses a ratchet in a single-piece with the barrel arbour between shoulders for guiding the arbour in the plate and the bridge. The barrel arbour includes a pivotal drive means for a core carrying the spring, with a drum and associated cover pivoting on the core and clamping the core with no axial play. The shake of the arbour is limited in one direction by the ratchet bearing on the bridge, and indirectly in the other direction by the core bearing, via a first end surface on an inner surface of the ratchet, with the opposite end surface bearing on the plate. This mechanism has the drawback of requiring a bridge to be removed for any repairs.
DE Patent No 1872388 in the name of EPPLE KG OTERO UHREN discloses a mechanism which is bulky in thickness and wherein shake is limited by a strip-spring cooperating with a groove in the arbour, between the bridge and the drum.
U.S. Pat. No. 1,110,061 in the name of KIENZLE discloses a barrel arbour capped with a core, the arbour including a groove parallel to its axis, in which a longitudinal pin acts as coupling/uncoupling cam between the core and the arbour according to the direction of pivoting of the arbour. The drum pivots on the core, as does the cover fixed to the drum. The length of the core just matches the distance of centres between the plate and the bridge, and the arbour can be fitted into the core after the lateral insertion of the assembly formed by the core, spring, drum and cover. The drum shake is limited by the bridge in one direction and by a stepped portion of the core in the other direction. Here again, limiting the shake of the arbour relative to the plate and to the bridge requires a flange to be added to the bridge to limit the travel of the ratchet, which is detrimental as regards the thickness of the mechanism.
CH Patent No 295135 in the name of BRAC AG discloses a smooth arbour including a head retained by a stepped portion of a centring sleeve of a plate, the other smooth end of the arbour pivoting in a bridge. A drum and a cover grip a core, which is coaxial with the arbour, with axial play and between two shoulders. On the smooth side of the arbour, the core carries a ratchet. The endshake of the core is limited in the direction in which the arbour head is supported in the centring sleeve by the ratchet bearing on a fillet of the bridge, and in the opposite direction by an end face of the core bearing on the centring sleeve. The centring sleeve represents additional thickness for the barrel.
U.S. Pat. No. 182,629 in the name of BARCLAY discloses a safety barrel of more complex execution, including an overload protection mechanism with radially moveable corners pushed by an additional spring.
FR Patent No 1473744 in the name of GHIELMETTI discloses a particular barrel, which includes a cover provided with slots cooperating with lugs of a non-metallic strip inserted between the spring and an auxiliary strip-spring.
These mechanisms either include a core of large diameter leaving little space for the spring, or shake-limiting components which are added to the stack of components required for operation of the barrel, and increase the total thickness of the barrel.